Abstract: A metrological instrument for measuring surface characteristics such as surface form of a component (300). The instrument includes a slideway (32) to cause relative movement between a surface sensing probe (84) and the component which allows the probe to traverse a surface of the component. A probe carriage (80) supports the probe to allow the probe to move as a body in a measurement direction relative to the carriage so as to follow a surface being sensed. An optical sensor (82) provides a signal indicative of the displacement of the probe relative to the probe carriage. A controller (83) acts in response to a signal from the optical sensor, to move the probe carriage so as to maintain the probe in an operational range. A further sensor (11) measures measurement of displacement of the probe in the measurement direction as it follows a surface during use of the instrument.